Graphic arts applications frequently require the accurate reproduction of a high resolution color image (commonly referred to as an "artwork"), such as a color photograph, a color drawing, a color layout and the like. A typical application might involve printing a high resolution color image or a series of such images on a page of a periodical, such as a magazine, or a corporate annual report.
In order to verify that a conventional color printing process generates a color halftone image that is an accurate reproduction of an original color artwork, a so-called "proof" image is generally made from a series of halftone separations of the artwork. The proof image is taken to be representative of the reproduced halftone image that will be generated by a multi-color printing press. Oftentimes, the proof image contains unexpected and unsightly Moire patterns, poor tone and/or color reproduction, or other artifacts which generally can be removed through appropriate rotation of a screen angle used in generating one or more of the separations or through appropriate color corrections. Once these adjustments are made, new separations are produced. Thereafter, the entire proofing process is iteratively repeated until a set of separations is found which will yield an acceptable depiction of the artwork.
The conventional iterative manual photographic process of producing an acceptable set of halftone separations, due to the inherent variability of the process, can be very tedious and inordinately time consuming. Unfortunately, in the graphic arts industry, publication deadlines are often extremely tight and afford very little, if any, leeway. Consequently, the available time in a graphic arts production environment allotted to a color technician to generate a set of halftone separations to meet a particular publication deadline, for example, is often insufficient to allow the technician adequate time, due to the trial and error nature of iterative process, to generate that set of separations which produces a very high quality halftone color image. As such, the technician is often constrained by time pressures to produce a set of separations that produces a visually acceptable, though not necessarily a very high quality, image.
In an effort aimed at reducing the time required and accompanying expense associated with the proofing process, the art has turned away from conventional manual photographic based proofing processes, particularly for use in high volume graphic arts applications, to other technologies. One of the first such technologies was electro-photography based proofing; an illustrative system employing this technology is described in U.S. Pat. No. 4,708,459 (issued to C. Cowan et al on Nov. 24, 1987 and assigned to the present assignee hereof--hereinafter referred to as the '459 Cowan et al patent). While this system does produce an excellent quality proof, this system appears to possess various limitations which, to a certain extent, have restricted its commercial attractiveness.
Specifically, many printing presses today utilize five or six differently colored inks. Typically, four of these inks are the primary colors: cyan, magenta, yellow and black (C,Y,M,K), with two additional inks being so-called "special" fifth and sixth colors. For the most part, printing presses fix the density at which a single colored halftone dot is printed, i.e. either the press deposits a fixed quantity of ink at a particular location on a page or not. As such, it has become apparent in the art for quite some time that a significant number of colors, such as for example phosphorescents (e.g. "hot pink"), pastels and golds, will fall outside the bounds of a gamut of colors that can be printed using only these four primary colors at fixed density levels. Accordingly, these colors, hereinafter referred to as "special" colors, can not be readily reproduced through the use of fixed density halftone printing using the four primary colored inks alone. As such, whenever such a special color is desired, an ink that has been specifically mixed to that color is generally used as either the fifth or sixth colored ink. Using that ink, the special color is printed in accordance with the halftone dot patterns on a corresponding separation for that color of an original artwork. In addition, from time to time, a printing job may call for an exact color, such as illustratively so-called "KODAK" yellow, that lies within the gamut associated with the four primary colors. ("KODAK" is a registered trademark of the Eastman Kodak Company of Rochester, New York.) However, to provide higher quality color reproductions and increased reliability of the color reproduction process than that generally obtainable through use of only fixed density primary color printing, a fifth or sixth ink that has been mixed to that specific color will be used, rather than the primary colors, to tone corresponding areas of each halftone reproduction.
In view of the use of five and six color halftone color printing, a concomitant need has arisen for some time in the art to be able to model both the fifth and sixth colors within a proof image. Generally speaking, this modelling could be accomplished through either of two approaches. First, one approach might be to produce a separate film within the proofing process for each special color that is to be used and then use an ink that has been matched to that color to tone corresponding halftone dot patterns therefor in the proof image. Unfortunately, this approach is likely to be quite expensive and cumbersome to implement and use. Fortunately, a reasonably close, rather than an exact, match is often adequate for use in analyzing a proof image for defects, particularly when a viewer is concerned with page layout, i.e. the areas on the proofed image in which corresponding portions of the replicated artwork are to appear, and the like. Accordingly, a second approach might be to produce a corresponding resultant color in some fashion through use of the four primary colors which, while being a reasonably close rather than an exact match to each special color, will nevertheless represent each of the special colors that is to be used.
The electro-photographic proofing system described in the '459 Cowan et al patent permits an operator to independently set the solid area density, over a reasonably wide range, of the halftone dots for each primary color. In this manner, dots of a different solid area density can be produced for each different primary color in a given proof image. Then, by accurately superimposing the primary colored halftone dots with different corresponding solid area densities in a common proof image, an extended range of colors, at least to a human observer, can be produced for use in simulating a special color(s). Unfortunately, in practice, this range, while greater than that obtainable without varying the solid area density between dots of different primary colors, nevertheless proved to be somewhat inadequate for use in modelling various special colors.
For a variety of reasons, such as for example, increased flexibility, control and throughput over that provided by electro-photographic proofing systems, the art is currently turning towards the use of so-called direct digital color proofing (DDCP) systems. These systems directly generate a halftone color proof image from a set of digitized continuous tone (contone) separations. Specifically, DDCP systems manipulate the separations in digital form to electronically generate appropriate halftone separations, including, inter alia, electronic screening and dot gain compensation, and then directly write the proof image using an appropriate marking engine (also referred to herein as a "proofing engine").
Therefore, a need presently exists in the art for a technique for inclusion in a DDCP system that can be used to model, not necessarily exactly, a wide range of special color(s) in a halftone color proof image through use of the available, illustratively four, different primary process colors (C,Y,M,K) and specifically halftone dots formed of these individual primary colors. Specifically, such a technique should significantly expand the range of special colors that can be modelled in a color proof image over that heretofore obtainable in the art, such as through electro-photographic technologies, as well as be compatible with digital marking engines.